This invention relates to the modulation of laser light and, more particularly, to Fabry-Perot modulators having a multiple quantum well modulator.
Asymmetric Fabry-Perot modulators (AFPM) have been utilized in the prior art for modulating laser light. The AFPM modulates a constant amplitude light by modulating the electro-absorption of a semiconductor material within an optical cavity defined by top and bottom reflectors. The optical cavity consists of a multiple quantum well semiconductor structure referred to as the electro-absorption material. By modulating the electric field across the electro-absorption material, the optical absorption of the semiconductor material varies and by that, so does the total reflectivity of the cavity. Specifically, unmodulated constant amplitude light that is incident upon the top reflector is partially reflected away from the electro-absorption material and partially transmitted through the electro-absorption material. The transmitted part is reflected by the bottom reflector, transmitted again through the electro-absorption material, and recombined with the original part of the incident light that was reflected by the top reflector. When the absorption in the electro-absorption material is low, the reflection of the incident light is dominated by the bottom reflectance and the total reflection is high. When the electro-absorption within the material is increased by increasing the applied field, the total reflectivity starts to decrease. When the absorption in each direction in the cavity, A(v), equals xc2xd (Rbxe2x88x92Rt), (where Rb is the reflectivity of the bottom reflector and Rt is the reflectivity of the top reflector), a cavity matching condition is achieved and the total reflectivity, Rtotal, equals zero.
Varying the electro-absorption between low (essentially zero) reflectivity at the matching condition-voltage and high reflectivity at low or zero voltage across the cavity therefore causes the magnitude of the reflect signal to be modulated in accordance with the modulating electronic field. When the top and bottom reflectivities and the absorption within the electro-absorption material are ideally chosen, the ratio between the amplitude of a xe2x80x9c1xe2x80x9d and a xe2x80x9c0xe2x80x9d pulse (referred to as the extinction or contrast ratio) is sufficiently large to ensure detection outside the AFPM.
Asymmetric Fabry-Perot modulators have been utilized in the prior art with a multiple quantum well (MQW) defined by a front reflector and a back reflector having a fixed cavity length L. The cavity length is chosen such that resonance occurs close to the long wavelength side of the unbiased MQW absorption edge so that application of a bias signal to the MQW causes the reflectivity of the fixed length cavity to become close to zero. Whitehead uses this arrangement to provide a high contrast modulator less sensitive to temperature variations and deviations from ideal reflectivities of the front and back reflectors than high-finesse Fabry-Perot modulators.
These AFPMs have reflecting cavities of a fixed length construction. As such, the parameters of the devices must be chosen carefully and must be fabricated with precision. This precise fabrication is necessary in order to match the optimal cavity length, cavity reflectivities and electro-absorption material with one another. These factors must also be maintained over the life of the AFPM devices. In addition, temperature variations may also alter the performance of AFPM devices.
As a result, one object of the present invention is to provide a novel asymmetric Fabry-Perot modulator having an adjustable cavity length, whereby the device may be tuned to a precise, absolute wavelength independent of an applied voltage across the electro-absorptive layer of the modulator.
Another object of the present invention is to provide a novel method for tuning an asymmetric Fabry-Perot modulator having an adjustable cavity length, whereby the device is tuned to an absolute wavelength independent of an applied voltage across the electro-optive layer of the modulator, and for thereafter keeping the cavity length tuned to that wavelength.
These and other objects are addressed by the present invention.
In one form of the invention, there is provided an asymmetric Fabry-Perot modulator having an adjustable cavity length for use in tuning the modulator length to a precise, absolute wavelength independent of an applied voltage across the electro-absorptive layer of the modulator, the modulator comprising first and second reflectors forming a resonant cavity therebetween; electro-absorption material disposed between the first reflector and second reflector, the absorption of the electro-absorption material being varied in response to an external modulating signal; and means for adjusting the length of said resonant cavity formed between the first reflector and the second reflector, the cavity length adjustment means include changing the position of the first reflector relative to the second reflector, wherein the magnitude of light output from said first reflector is determined by the reflectivity of the first reflector, the absorption in the electro-absorption material and the length of the resonant cavity as defined by the cavity length mirror adjustment means.
In another form of the invention, there is provided a method for tuning an asymmetric Fabry-Perot modulator, the method comprising the steps of: (1) reflecting laser light between a first reflector and a second reflector forming a resonating cavity therebetween; (2) positioning the first reflector to change the length of the resonant cavity to an optimal wavelength as defined by the properties of electro-absorption material contained in the modulator; and (3) monitoring the output of the first reflector as the resonant cavity length is tuned whereby to tune the Fabry-Perot modulator to the desired wavelength.